Much of the mechanics (and some of the background info) is based heavily around a game I've run at ACNW called Pulp Chaos, which in turn stole a number of ideas for an assortment of people. Technically speaking, I consider this game to be set in the same universe as Pulp Chaos, though the games are unlikely to have any interaction.

Much of the Rebman cosmology and setting is stolen heavily from the never-published Rebma Sourcebook. The original sourcebook proposed an aquatic version of the Lords of Chaos called the Tritons. They had their own power (the Vortex) and their own version of the Courts of Chaos (the Maelstrom). I have used many concepts and characters originally introduced in that book in this game, though I've tweaked some of the ideas to fit with the cosmology for this game a bit better. For another take on the material presented there, as well as a summary of the rules, you can check out the information I'd posted for a game I'd run several years ago called Free Rebma.

Though this would probably better fit underneath the Background Information section, different games tend to handle this aspect of Rebma differently and it is such a prominent part of playing in Rebma that the approach this game uses bears explanation up front.

Within the city limits and along the stairway down to Rebma, the water functions mostly like air. There are a few cosmetic differences: conversation sounds a little more distant and bubbles occasionally appear when you talk, but otherwise it can be treated like “air.” You can walk along the sea floor without trouble, fire still burns, you can drink a glass of wine, you can swing a sword without weird problems with water resistance, you can bring a book down to read and it won’t turn to mush, etc.

But with all this, you can still swim through the water and fish can still breathe within the city limits as well. Most people still walk, since it takes more energy to swim and for some they don’t want to risk reaching the upper limit of the city’s enchantment on accident. The magical effect that grants this dual state stops immediately past the walls to the city and about 200 feet above the sea floor.

The aquatic predators that prowl the waters around Rebma are a constant threat to the denizens of Rebma. From the steps of the Faiella-Bionin to the royal palace, the patrols are vigilant in fighting off any threats that may harm the nobility of Rebma. As one gets farther seaward past the palace, the patrols become less and less reliable. This crescent along the far edge of the city is commonly referred to as the Undertow. It is there that some of the poorest denizens of Rebma live. (At least, the poorest ones that can’t breathe water.)
Because of the reduced security, this is also a common point of entry (and exit) for most of the aquatic smugglers. Combined with the rampant poverty, this is where most of the black markets, red light districts and organized crime in the city can be found. It’s also a good place to get killed.

Here are some notable characters and features.

Commander Gwenyth ab Lowri: The highest ranking member of the constabulary in Undertow.

Fat F’thul: One of several crime lords prominent in Undertow. It identifies with no gender and has best been described as resembling a sea anenome mounted on the back of a crab. It hails from somewhere along the Coral Branch and is now in the “import/export” business, mainly shuttling contraband in and out of the city and maintains a small office in a warehouse.

Awena, the Witch Queen: Another of the crime lords, her scales and fish-like facial features denote a stronger connection to triton lineage than other Rebmans. It is said that she is also a fierce practitioner of the Old Ways. She works out of her shop, which sells an assortment of protective charms and similar arcane materials.

The Flotsam: One of many seedy bars in Undertow. It is owned by an eel-like creature named Stan.

No Survivors: Another treacherous watering hole, it’s run by a blind and half-mad Ri’ikan who served in Corwin’s fleet in the attack on Amber and survived falling overboard and sinking down into Rebma.

Instructions: Come up with a concept. Pick an occupation, a primary attribute, and a spiff. If you do not take Adaptation you will need to decide if your character is an air-breather or a water-breather. If you get your character in before the convention, you may have two spiffs. Race is a little fast and loose, and is largely reflected in the Spiffs you take. A typical Rebman might have Adaptation. A squamous monstrosity from the depths might have “Minor Shapeshifting” or “Ace in the Hole.” Again: This is all pretty fast and loose.

Occupations

Free Agent: Odd-job man for hire. Sometimes someone needs a bodyguard. Sometimes they need an investigator. Sometimes they just need a message delivered. You’re the person they come to. This is the Rebman version of the P.I. or consulting detective.

Spy: Maybe you are in Rebma posing as a merchant. Maybe you’re just there as part of a consulate. Maybe you are an agent of a House that wants to keep tabs on the underworld. Maybe you’re a local who’s willing to sell out his homeland for a few extra ducats. Either way your loyalties (and your income) are tied to somewhere outside of the Undertow. This could be as close as Rebma or as far away as the Courts of Chaos.

Constable: You work for the law enforcement agency of Rebma. Depending on your spiffs, this could represent any of a number of law enforcement jobs, from a jackbooted thug to a special investigator.

Criminal: You make your living by subverting the laws of the Crown. You may be a leg breaker for a crime lord. You could be an information broker running out of the back of a bar. It largely depends on the spiffs you choose.

Attributes

This game uses, to a limited degree, the four standard ADRPG attributes. Instead of having a point value, there are a series of ranks:

Mook: On par with a standard “human.”

Gritty: A talented human.

Hard-Boiled: A very talented human.

Untouchable: High end of human ability.

Each PC starts out at Mook level, and may upgrade one attribute to Gritty for free. Additional improvements to attributes may be made by spending an appropriate Spiff.
The four attributes are interpreted differently from the rule book:

Pysche: This does not provide telepathy through eye contact or physical contact. Nor does it provide psychic manipulation, surgery or even “mind raping.” At best, you may be able to daze or stun someone over a Trump contact. On the other hand, this attribute covers social interaction and stealth. Magical abilities depend heavily on this ability. You must have at least a Gritty Psyche to activate a Trump or wrought mirror.

Warfare: Most of the physical aspects of this attribute have been stripped out. This attribute mainly covers tactical reasoning, ranged combat and most formal dueling situations. You can, in theory, use this with something like a sword or staff to buy a bit of time and moving room. However, a significant Strength advantage negates the usefulness of such a tactic.

Strength: This covers most physical abilities. Running, jumping, beating someone down. In hand-to-hand combat, this attribute is more heavily weighted than Warfare.

Endurance: This functions about the same as in the book. While not everyone necessarily has the extreme regenerative ability typical of a normal “Amber” character, this game will likely not last long enough for that to be relevant.

Spiffs

Ace in the Hole: You have some sort of special attack: a poisonous bite, diamond-hard claws, the ability to breath fire, a dart-launching parasite lodged in your abdomen. This is much more potent than fighting with your bare knuckles. This could, alternately, represent some other special racial ability. Like aquatic adaptation, fire resistance, etc.

Adaptation: Though others besides Rebmans can easily have this, and not all Rebmans have it, this Spiff could be considered the “Rebman racial pacakge.” It grants the ability to breathe underwater, swim efficiently, endure the cold and extreme pressures of the deep ocean, see in minimal light, hear well, direction sense and a limited motion sense. Possession of this spiff among Rebmans further represents a stronger connection to the old blood of Rebma.

Dirty Bastard: Terms of engagement are for pussies. You gain an extra edge in stand-up fights courtesy of knives up your sleeves, knees to the crotch or some other skullduggery. You are at your best, though, when it comes to stabbing people in the back or shooting them from hiding.

Don’t Get Cute: You are intimidating. While some people use threats of violence to get what they want, you can just lay down a mean stink-eye and people will stop in their tracks or start blabbing information. This only works if you have a higher Psyche than your opponent.

Duelist: You are an expert at some traditional dueling weapon or combination of dueling weapons: rapier and main-gauche, trisp and fandon, pistols, etc. While using that weapon, you are deadly in most situations and almost unstoppable in a formal duel. You likely are the type to pose as a stand-in combatant when a duel of honor is to occur.

Dumb Luck: What you lack in skill and ability, you make up for in absurd good fortune.
Empath: This is low end psychic ability. It grants the ability to sense nearby minds, read emotions, detect auras and, with a significant advantage in Psyche, read surface thoughts.

Goody: You possess a magical item of moderate potency. A Trump deck, an enchanted trident, a wroght mirror, etc. Alternately, this could be a creature such as a guard animal or a familiar.

Hedge Mage: You dabble in an assortment of magics and possess and eclectic blend of sorcery and conjuration. The exact angle of your magical ability depends on your profession. You also possess an item that allows you to rack 3 spells within it. Note that this less resembles the Amber Diceless sorcery rules, and more of a gritty, organic low-powered approach to magic.

Limited Shapeshifter: You have one alternate form and can sculpt your features. Your alternate form can have some perk that your normal form does not have such as armor, venomed claws, or some other nifty ability.

Made Man: You work for a local crime lord, and may be able to draw upon his resources in times of need. (Assuming it’s worth his time.)

Minion: You have a person that is loyal to you and does your work. He may have one Spiff.
Minor Noble: You are the extra, unwanted child of a noble house. This noble line could be associated with Rebma, Amber or even Chaos. You could even be of the Amberite royal blood. You do not have any power derived from this. You are, after all, slumming in Rebma for a reason. But in times of need you can contact your family and ask them for help.

Nothing Up Your Sleeve: You are a master of sleight of hand. Picking pockets, cup-and-ball games and common parlor tricks are all within your realm of mastery.

Talented: You may increase an attribute by another notch. This may be one that you’ve already increased, or a different one.

Tough as Nails: Whether it’s due to an armored hide, uncanny resilience, alien anatomy or dumb luck, you can take a lickin’ and keep on tickin’. You can typically resist the equivalent of a gunshot wound or two and still be okay. For some characters, this may simply mean they are very good at avoiding damage.

Vet: You served in the armies of Amber or Chaos and fought in the Patternfall War. As such, you have seen much of Shadow. You are able to speak a smattering of languages throughout Shadow (and all the dirty words of even more), have a very good sense of Shadow geography, and still have friends in the military that you draw upon in times of need.

Well-Connected: You have extensive contacts throughout Rebma. They may not necessarily be interested in actively helping, but they might be able to provide information.

The Old Blood: A common point of distinction between the denizens of Rebma is how strong their connection to their triton ancestors are. More the most part this is simply visible through greenish tones to the skin and the ability to breathe underwater, though in some rare instances more animalistic features do manifest: scales, shark-like teeth, fins, etc. What is disputed, however, is how much weight to give such a lineage. Some in Rebma try to distance themselves from their more alien predecessors while others feel that a return to the Old Ways would be best.

Nobility: Since the departure of the tritons and the ascension of Moins to the throne, Rebma has become a strong matriarchy. In part this is due to strong female leadership in the kingdom, but also due to a desire to better track strengthen the Old Blood.

Royal Family: Tracing their lineage back to Lir, the royal family possess the most influence in the kingdom. In addition to Queen Moire, the notable members of the royal family include:

Princess Llewella, half sister to Moire

Princess Scylla, sister to Moire

Prince Martin, grandson of Moire

Lady Dayle, daughter of Llewella

Lord Oswald, son of Scylla

Major Houses: Tracing connection to the former triton lords, these houses pride themselves on their good breeding and the strength of the Old Blood. Borrowing from Amber’s nobility, they are typically considered duchies and marquisates. Most of them have lands, but pedigree counts for a good deal more. There are roughly a dozen major Houses, including the royal family.

Minor Houses: Though some of these houses have a strong streak of Old Blood, their primary distinction is that they have earned the good favor of the Crown. There are perhaps fifty or so minor houses, the heads of which bear titles such as countess, viscountess and baroness.

The Church of the Unicorn: Though jokingly referred to as the Church of the Sea-Horse, this institute represents one of several footholds Amber has within Rebma. Though primarily associated with expatriate Amberites, many self-described “forward-thinking” Rebman’s have taken up faith in their neighbor’s religion.

The Unicorn Templars: The militant wing of the Church of the Unicorn. Though they do not have an official presence in Rebma, they are sometimes active.

The Church of the Serpent: The official religion of Chaos has only a small temple here, mainly catering to the faithful Chaosian emigrants that have settled in Rebma.

The Serpent Guard: The militant wing of the Church of the Serpent. They almost never have anything to do with Rebma, but only “almost never.”

The Old Faith/The Old Ways: Prior to the departure of the tritons, this was the primary religion of Rebma. Centered primarily on a deity known as “Leviathan,” this was primarily a loosely organized animistic faith. Though Lir was the nominal head of the faith, the practice was a very personal experience. In the end, each believer faces the darkness of the depths alone. Though it is not an openly practiced faith, it is still prevalent in Rebman culture. Conservative and old-fashioned Rebmans become fairly militant on the subject. Some become deeply involved with the faith, often blending sorcery and other powers with their faith. The polite call them priests. The less polite call them witches.

The Constabulary: Tasked with enforcing the law and investigating criminal infractions in the city, each neighborhood in Rebma has a Commander overseeing the day to day work, with the Lady High Constable representing the highest authority in that regard under the Queen.

Rebman Army: The standing army of Rebma. Membership typically requires a good measure of the Old Blood or else an aquatic existence in order to travel outside of the city.

Rebman Navy: Though it may seem a bit silly when initially described, Rebma does have a small fleet to patrol the surface of the water above their city.

Free Agents: This describes a broad category of employment in Chaos, representing a mix of private investigator, bravo and bounty hunter. Free agents differ from mercenaries in that they don’t function in any militaristic capacity.

Mirrorwrights: Rebma has their own equivalent to trump artistry called “mirrorwrighting,” introduced originally by the tritons. The process involves enchanting a mirror during its creation. Once done, it becomes capable of opening a communication link with other mirrors. As long as you can visual another wrought mirror you have seen (and you have a high enough Psyche) you can open up communication with that mirror through one of those enchanted mirrors and, if the mirror is large enough, even travel through one. It is said that mirrorwrights can also temporarily enchant mundane mirrors or even access mirrors that are not enchanted, in addition to other abilities.

Places

Castle Rebma: The royal palace of Rebma, located near the center of the city.

Lir College: The school of higher learning within Rebma.

The Depths: Not far from the city of Rebma is a deep-sea trench that is usually just referred to as “the Depths.” It is truly the most hard scrabble location in Rebma, accessible only to those who can breathe underwater. The Rebman government almost never gets involved in this area. It’s a lawless land filled with predators, both sentient and otherwise.

Sargasso Sea: The aquatic counterpart to the Dancing Mountains, this represents the area where the shadows of Amber and Chaos overlap. The Sargasso sea represents a seaweed entangled area of overlapping Shadows often trap the unwary.

The Coral Branch: Those Shadows that Rebma has regular trade with.

Things

Underwater Survival Equipment: For those air-breathers who want to venture outside of the city, there is an assortment of options available. Most options involve some sort of arcane equipment, potion or the like enchanted to provide the ability to breathe outside of the city and resist the cold and pressure. On rare occasion, diving equipment occasionally does make its way into the city: Diving bells, armored diving suits, and scuba tanks all occasionally appear in marketplaces around the city.

Wrought Mirrors: The enchanted mirrors created by mirrorwrights. Assuming you have sufficient Psyche, you can use one. You can use one to contact any other wrought mirror you have seen. You can also lock a mirror against contact in order to prevent unwanted calls or spying.